The present invention relates to a white balance adjusting device for a video camera and to an improvement of response characteristics thereof to variation of relative moving speed of an object in a view angle of the video camera.
In a conventional device such as shown in Japanese patent application laid-open No. 219291/1988 or U.S. Pat. No. 4,797,733, video signal from a portion in a video image, which can be considered as having white color or having no color, is extracted and gains of red (R) and blue (B) components of the video signal are controlled such that a ratio between R and B gains of an averaged signal of this portion becomes 1 while following a locus of color temperature variation of blackbody radiation. In such conventional device, control signal for adjusting white balance is generated on a basis of comparison of the average signal value of the white portion with a reference signal value corresponding to R-B(=0) in color temperature.
This is effective to automatically adjust white balance in a case where there is a white color or no color portion. In a case where there is no such portion within the image, white balance is fixed in a reference regulation state. The reference regulation is set in a range 4000.degree.-5000.degree. K in view of a variation range of color temperature under practical illumination. In such case where there is no white portion in the image and an automatic adjustment can not be performed, error may be increased dependent on lighting condition.
As to a response characteristics of white balance adjustment to color temperature variation of illumination with time, Japanese patent application laid-open No. 109686/1988 discloses a technique in which it is fixedly determined by a time constant of a color-difference signal integrating, low-pass filter of a white balance adjusting circuit and a pulse period of counter driving pulse.
In this case, a compatibility of high response to abrupt change of illumination and stability against colored portion incoming to the image is hardly obtained. That is, when adjusting speed is increased to improve only response characteristics, sensitivity to panning and/or moving object becomes too high, causing stability to be degraded. On the other hand, when adjusting speed is reduced to improve stability, smoothness response to a change of lighting condition of an object from high illumination to low illumination can not be obtained.
An object of the present invention is to reduce error generation in white balance adjustment even in a case where there is no white portion in an image.
Another object of the present invention is to provide a device which can extract only white portion reliably to allow an accurate white balance adjustment.
Another object of the present invention is to provide a white balance adjusting device which can perform a white balance adjustment even with artificial illumination such as fluorescent light which does not correspond to blackbody radiation.
Another object of the present invention is to provide an automatic white balance adjusting circuit which can provide compatibility of stability in usual lighting condition and high response to illumination change by selectively using low speed response and high speed response.